Generally, a motor consumes electric power during acceleration and operates as a generator during deceleration since it is necessary to reduce an induced electromotive force generated during rotation of the motor. For the sake of convenience in the following description, an acceleration operation of a motor will be referred to as “motor power running” or simply “power running”, and a deceleration operation thereof will be referred to as “motor regeneration” or simply “regeneration”.
In industrial machines such as machine tools, manufacturing machines, and robots, a motor is mounted on a drive shaft (hereinafter this configuration is referred to as “shaft configuration”), and the drive shaft is driven by the motor. As contrasted with a motor applied to such an industrial machine, a motor control device for controlling the motor is configured to include a rectifier device and a motor drive device. The rectifier device converts an alternating-current voltage applied from an alternating-current power supply as an input power supply into a direct-current voltage. The motor drive device converts the direct-current voltage converted by the rectifier device into an alternating-current voltage and applies the alternating-current voltage to a motor as an object to be controlled to perform variable speed control of the motor.
The rectifier device is generally configured to include a power module and a smoothing capacitor. The power module includes a bridge rectifier circuit constituted by a rectifier element such as a diode. The smoothing capacitor smooths output of the power module. The alternating-current voltage from the input power supply is applied to an alternating-current input terminal of the power module, and is subjected to alternating-current to direct-current conversion by the power module, and a direct-current voltage obtained by the alternating-current to direct-current conversion is smoothed by the smoothing capacitor. Therefore, the rectifier device includes a direct-current terminal for applying the smoothed direct-current voltage to the motor drive device.
During motor power running, an input voltage is converted to a direct-current voltage through alternating-current to direct-current conversion by the rectifier device and the direct-current voltage is applied to the motor drive device via the smoothing capacitor. Then, the direct-current voltage from the smoothing capacitor is subjected to direct-current to alternating-current conversion by the motor drive device, an alternating-current voltage obtained by the conversion is applied to the motor, and thereby the motor is driven.
During motor regeneration, induced electromotive force generated by the motor (hereinafter referred to as “regenerative power”) is subjected to alternating-current to direct-current conversion by the motor drive device, and a direct-current voltage obtained by the conversion is applied to the smoothing capacitor. Therefore, in a case where the regenerative power of the motor is large, an inter-terminal voltage of the smoothing capacitor increases, and when the inter-terminal voltage exceeds an allowable voltage of the smoothing capacitor or an allowable voltage of the power module, the smoothing capacitor or the power module may be damaged.
Examples of a method for recovering the regenerative power generated during motor regeneration as described above include a resistance regeneration method in which regenerative power is consumed as heat by a resistor, a capacitor regeneration method in which regenerative power is stored in a capacitor, and a power regeneration method in which regenerative power is returned to an input power supply. In recent years, there is a trend of energy saving in industrial machines such as those described above, and an increasing number of rectifier devices, to which the power regeneration method is applied, are adopted.
The rectifier device to which the power regeneration method is applied is a rectifier device to which a power module capable of performing mutual conversion of power, that is, alternating-current to direct-current conversion and direct-current to alternating-current conversion, by a plurality of rectifier elements and a plurality of switching elements is applied as a power module. That is, the rectifier device to which the power regeneration method is applied can operate, during motor power running, as an alternating-current to direct-current conversion device to supply power necessary for driving the motor to the motor drive device via the smoothing capacitor, and can operate, during motor regeneration, as a direct-current to alternating-current conversion device to return regenerative power of the motor to the input power supply via the smoothing capacitor.
Examples of a control method of a rectifier device to which the power regeneration method is applied include a PWM regenerative converter method using PWM control and a 120-degree conduction regeneration method. The PWM regenerative converter method can make a current from the input power supply a sinusoidal wave. However, since a PWM operation is performed regardless of whether during motor power running or motor regeneration, there occur an increase in heat generated in the power module due to switching losses, and a following increase in size of a cooling mechanism, which results in a disadvantage that a casing itself increases in size. In addition, switching noise increases with the PWM operation, which necessitates addition of an input filter or the like to suppress the switching noise, and thereby cost increases, in general.
On the other hand, in the rectifier device to which the power regeneration method of the 120-degree conduction regeneration method is applied, a phase (hereinafter appropriately referred to as “voltage phase”) of a voltage applied from the input power supply (hereinafter appropriately referred to as “power supply voltage”) is detected and power is regenerated to the input power supply only in a section of 120 degrees of the power supply voltage. In the 120-degree conduction regeneration method, switching operations of switching elements are required only at the start and the end of the 120-degree section, and the switching losses can be greatly reduced as compared with the PWM converter method. In addition, since the number of switching operations is small, switching noises are reduced as well, and the rectifier device can be configured at lower cost than in the PWM converter method. While the switching operation is always necessary in the PWM converter method, in the 120-degree conduction regeneration method, a power regenerative operation by the switching operation is stopped and an alternating-current to direct-current conversion is performed in a rectifier bridge circuit of the power module during motor power running, and thereby it is possible to reduce the switching losses of the switching elements. For this reason, in industrial machines such as those described above, the rectifier devices to which the power regeneration method of the 120-degree conduction regeneration method is applied are adopted in many cases. For the sake of convenience in the following description, the rectifier device to which the power regeneration method is applied is referred to as “power regenerative converter”.
In industrial machines such as those described above, when a shaft configuration includes a plurality of motors, a plurality of motor drive devices is required. On the other hand, one power regenerative converter is usually provided in order to save a space for a control panel on which a motor control device is disposed and to reduce cost thereof. That is, in a general configuration, only one power regenerative converter is provided for a plurality of motor drive devices.
Output power of the power regenerative converter is determined by power supplied to a motor by a motor drive device to be connected thereto, that is, output of the motor. Therefore, when the output of the motor driven by the motor drive device is large, power supplied by the power regenerative converter increases, and a large current flows through a power module mounted inside the power regenerative converter.
Regarding allowable output power of the power regenerative converter, there are allowable continuous rated output capacity and allowable maximum output capacity. The allowable continuous rated output capacity represents power which the power regenerative converter can continuously supply to the motor drive device, and the allowable maximum output capacity represents the maximum power which the power regenerative converter can supply.
In the motor control device to which the power regenerative converter is applied, when the motor performs an operation in which the allowable output power is exceeded and a state where allowable supply output power of the power regenerative converter is exceeded is continued, life degradation of the power regenerative converter occurs, and there may be damage thereof in some cases.
Therefore, in motor control devices used for industrial machines, a power regenerative converter is selected based on the continuous rated output of each of the motors and the maximum output of each of the motors. Specifically, a total of the continuous rated output and a total of the maximum output of each of the motors are calculated, and a power regenerative converter is selected of which the totals will be within the allowable continuous rated output capacity and within the allowable maximum output capacity, respectively.
When the selection of the power regenerative converter is performed as described above, it is possible to prevent the power regenerative converter from being brought into an overload condition, and to prevent life degradation and damage of the power regenerative converter. On the other hand, with such a selection method, even in a case where the total of the continuous rated output of each of the motors is within the allowable continuous rated output capacity, when the total of the maximum output of each of the motors exceeds the allowable maximum output capacity, a power regenerative converter with large capacity is selected. In addition, even in a case where the total of the maximum output of each of the motors is within the allowable maximum output capacity, when the total of the continuous rated output of each of the motors exceeds the allowable continuous rated output capacity, a power regenerative converter with large capacity needs to be selected similarly, which increases the size of a control panel, and may lead to an increase in the cost of the motor control device.
Generally, a protection device such as a breaker, an electric wire used as a power line for connecting an input power supply and a power regenerative converter, and ancillary devices such as a transformer disposed on the input power supply side to secure power supply capacity, are selected based on the capacity of the selected power regenerative converter and are determined by the allowable continuous rated output capacity of the power regenerative converter. When a power regenerative converter with large capacity is selected, a breaker or a transformer with large capacity is selected and a power line with a large wire diameter is used accordingly, which leads not only to an increase in the cost of the motor control device but also to an increase in the cost of the industrial machine as a whole.
In the selection of the power regenerative converter described above, values preset by a manufacturer which provides the motor control device are generally used as the continuous rated output of the motor and the maximum output of the motor. Consequently, there is a high possibility that a power regenerative converter having an excessive margin is set. For example, in a case of a machine tool including a plurality of servo motors and a spindle motor, there are few cases where the maximum output operations of all motors overlap. In a machine tool, servo motors operate with continuous rated output in few cases, and therefore, it is expected that capacity of the power regenerative converter will be larger in comparison with an actual operation of each of motors in many cases.
However, when each of the motors performs an unexpected operation and the sum of the rated continuous output of all motors or the sum of the maximum output of all the motors exceed the allowable output power capacity of the power regenerative converter (hereinafter referred to as “overload condition”), the power regenerative converter may suffer adverse effects. Therefore, the above-described selection method of the power regenerative converter is adopted in order to prevent problems from occurring even when there is an unexpected operation, which hinders cost reduction of industrial machines.
Regarding such problems, Patent Literature 1 below discloses a technique in which an alternating current flowing through an input side of a rectifier device is monitored, and when the alternating current is outside a range of a predetermined determination value, alternating-current power supplied by a motor drive device is controlled so that a motor operates in accordance with a torque command which is further limited than a torque command defined by a motor operation command.